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<FONT color="green">001</FONT>    /*<a name="line.1"></a>
<FONT color="green">002</FONT>     * Licensed to the Apache Software Foundation (ASF) under one or more<a name="line.2"></a>
<FONT color="green">003</FONT>     * contributor license agreements.  See the NOTICE file distributed with<a name="line.3"></a>
<FONT color="green">004</FONT>     * this work for additional information regarding copyright ownership.<a name="line.4"></a>
<FONT color="green">005</FONT>     * The ASF licenses this file to You under the Apache License, Version 2.0<a name="line.5"></a>
<FONT color="green">006</FONT>     * (the "License"); you may not use this file except in compliance with<a name="line.6"></a>
<FONT color="green">007</FONT>     * the License.  You may obtain a copy of the License at<a name="line.7"></a>
<FONT color="green">008</FONT>     *<a name="line.8"></a>
<FONT color="green">009</FONT>     *      http://www.apache.org/licenses/LICENSE-2.0<a name="line.9"></a>
<FONT color="green">010</FONT>     *<a name="line.10"></a>
<FONT color="green">011</FONT>     * Unless required by applicable law or agreed to in writing, software<a name="line.11"></a>
<FONT color="green">012</FONT>     * distributed under the License is distributed on an "AS IS" BASIS,<a name="line.12"></a>
<FONT color="green">013</FONT>     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.<a name="line.13"></a>
<FONT color="green">014</FONT>     * See the License for the specific language governing permissions and<a name="line.14"></a>
<FONT color="green">015</FONT>     * limitations under the License.<a name="line.15"></a>
<FONT color="green">016</FONT>     */<a name="line.16"></a>
<FONT color="green">017</FONT>    package org.apache.commons.lang3.concurrent;<a name="line.17"></a>
<FONT color="green">018</FONT>    <a name="line.18"></a>
<FONT color="green">019</FONT>    /**<a name="line.19"></a>
<FONT color="green">020</FONT>     * &lt;p&gt;<a name="line.20"></a>
<FONT color="green">021</FONT>     * This class provides a generic implementation of the lazy initialization<a name="line.21"></a>
<FONT color="green">022</FONT>     * pattern.<a name="line.22"></a>
<FONT color="green">023</FONT>     * &lt;/p&gt;<a name="line.23"></a>
<FONT color="green">024</FONT>     * &lt;p&gt;<a name="line.24"></a>
<FONT color="green">025</FONT>     * Sometimes an application has to deal with an object only under certain<a name="line.25"></a>
<FONT color="green">026</FONT>     * circumstances, e.g. when the user selects a specific menu item or if a<a name="line.26"></a>
<FONT color="green">027</FONT>     * special event is received. If the creation of the object is costly or the<a name="line.27"></a>
<FONT color="green">028</FONT>     * consumption of memory or other system resources is significant, it may make<a name="line.28"></a>
<FONT color="green">029</FONT>     * sense to defer the creation of this object until it is really needed. This is<a name="line.29"></a>
<FONT color="green">030</FONT>     * a use case for the lazy initialization pattern.<a name="line.30"></a>
<FONT color="green">031</FONT>     * &lt;/p&gt;<a name="line.31"></a>
<FONT color="green">032</FONT>     * &lt;p&gt;<a name="line.32"></a>
<FONT color="green">033</FONT>     * This abstract base class provides an implementation of the double-check idiom<a name="line.33"></a>
<FONT color="green">034</FONT>     * for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd<a name="line.34"></a>
<FONT color="green">035</FONT>     * edition, item 71. The class already implements all necessary synchronization.<a name="line.35"></a>
<FONT color="green">036</FONT>     * A concrete subclass has to implement the {@code initialize()} method, which<a name="line.36"></a>
<FONT color="green">037</FONT>     * actually creates the wrapped data object.<a name="line.37"></a>
<FONT color="green">038</FONT>     * &lt;/p&gt;<a name="line.38"></a>
<FONT color="green">039</FONT>     * &lt;p&gt;<a name="line.39"></a>
<FONT color="green">040</FONT>     * As an usage example consider that we have a class {@code ComplexObject} whose<a name="line.40"></a>
<FONT color="green">041</FONT>     * instantiation is a complex operation. In order to apply lazy initialization<a name="line.41"></a>
<FONT color="green">042</FONT>     * to this class, a subclass of {@code LazyInitializer} has to be created:<a name="line.42"></a>
<FONT color="green">043</FONT>     *<a name="line.43"></a>
<FONT color="green">044</FONT>     * &lt;pre&gt;<a name="line.44"></a>
<FONT color="green">045</FONT>     * public class ComplexObjectInitializer extends LazyInitializer&amp;lt;ComplexObject&amp;gt; {<a name="line.45"></a>
<FONT color="green">046</FONT>     *     &amp;#064;Override<a name="line.46"></a>
<FONT color="green">047</FONT>     *     protected ComplexObject initialize() {<a name="line.47"></a>
<FONT color="green">048</FONT>     *         return new ComplexObject();<a name="line.48"></a>
<FONT color="green">049</FONT>     *     }<a name="line.49"></a>
<FONT color="green">050</FONT>     * }<a name="line.50"></a>
<FONT color="green">051</FONT>     * &lt;/pre&gt;<a name="line.51"></a>
<FONT color="green">052</FONT>     *<a name="line.52"></a>
<FONT color="green">053</FONT>     * Access to the data object is provided through the {@code get()} method. So,<a name="line.53"></a>
<FONT color="green">054</FONT>     * code that wants to obtain the {@code ComplexObject} instance would simply<a name="line.54"></a>
<FONT color="green">055</FONT>     * look like this:<a name="line.55"></a>
<FONT color="green">056</FONT>     *<a name="line.56"></a>
<FONT color="green">057</FONT>     * &lt;pre&gt;<a name="line.57"></a>
<FONT color="green">058</FONT>     * // Create an instance of the lazy initializer<a name="line.58"></a>
<FONT color="green">059</FONT>     * ComplexObjectInitializer initializer = new ComplexObjectInitializer();<a name="line.59"></a>
<FONT color="green">060</FONT>     * ...<a name="line.60"></a>
<FONT color="green">061</FONT>     * // When the object is actually needed:<a name="line.61"></a>
<FONT color="green">062</FONT>     * ComplexObject cobj = initializer.get();<a name="line.62"></a>
<FONT color="green">063</FONT>     * &lt;/pre&gt;<a name="line.63"></a>
<FONT color="green">064</FONT>     *<a name="line.64"></a>
<FONT color="green">065</FONT>     * &lt;/p&gt;<a name="line.65"></a>
<FONT color="green">066</FONT>     * &lt;p&gt;<a name="line.66"></a>
<FONT color="green">067</FONT>     * If multiple threads call the {@code get()} method when the object has not yet<a name="line.67"></a>
<FONT color="green">068</FONT>     * been created, they are blocked until initialization completes. The algorithm<a name="line.68"></a>
<FONT color="green">069</FONT>     * guarantees that only a single instance of the wrapped object class is<a name="line.69"></a>
<FONT color="green">070</FONT>     * created, which is passed to all callers. Once initialized, calls to the<a name="line.70"></a>
<FONT color="green">071</FONT>     * {@code get()} method are pretty fast because no synchronization is needed<a name="line.71"></a>
<FONT color="green">072</FONT>     * (only an access to a &lt;b&gt;volatile&lt;/b&gt; member field).<a name="line.72"></a>
<FONT color="green">073</FONT>     * &lt;/p&gt;<a name="line.73"></a>
<FONT color="green">074</FONT>     *<a name="line.74"></a>
<FONT color="green">075</FONT>     * @since 3.0<a name="line.75"></a>
<FONT color="green">076</FONT>     * @version $Id: LazyInitializer.java 1088899 2011-04-05 05:31:27Z bayard $<a name="line.76"></a>
<FONT color="green">077</FONT>     * @param &lt;T&gt; the type of the object managed by this initializer class<a name="line.77"></a>
<FONT color="green">078</FONT>     */<a name="line.78"></a>
<FONT color="green">079</FONT>    public abstract class LazyInitializer&lt;T&gt; implements ConcurrentInitializer&lt;T&gt; {<a name="line.79"></a>
<FONT color="green">080</FONT>        /** Stores the managed object. */<a name="line.80"></a>
<FONT color="green">081</FONT>        private volatile T object;<a name="line.81"></a>
<FONT color="green">082</FONT>    <a name="line.82"></a>
<FONT color="green">083</FONT>        /**<a name="line.83"></a>
<FONT color="green">084</FONT>         * Returns the object wrapped by this instance. On first access the object<a name="line.84"></a>
<FONT color="green">085</FONT>         * is created. After that it is cached and can be accessed pretty fast.<a name="line.85"></a>
<FONT color="green">086</FONT>         *<a name="line.86"></a>
<FONT color="green">087</FONT>         * @return the object initialized by this {@code LazyInitializer}<a name="line.87"></a>
<FONT color="green">088</FONT>         * @throws ConcurrentException if an error occurred during initialization of<a name="line.88"></a>
<FONT color="green">089</FONT>         * the object<a name="line.89"></a>
<FONT color="green">090</FONT>         */<a name="line.90"></a>
<FONT color="green">091</FONT>        public T get() throws ConcurrentException {<a name="line.91"></a>
<FONT color="green">092</FONT>            // use a temporary variable to reduce the number of reads of the<a name="line.92"></a>
<FONT color="green">093</FONT>            // volatile field<a name="line.93"></a>
<FONT color="green">094</FONT>            T result = object;<a name="line.94"></a>
<FONT color="green">095</FONT>    <a name="line.95"></a>
<FONT color="green">096</FONT>            if (result == null) {<a name="line.96"></a>
<FONT color="green">097</FONT>                synchronized (this) {<a name="line.97"></a>
<FONT color="green">098</FONT>                    result = object;<a name="line.98"></a>
<FONT color="green">099</FONT>                    if (result == null) {<a name="line.99"></a>
<FONT color="green">100</FONT>                        object = result = initialize();<a name="line.100"></a>
<FONT color="green">101</FONT>                    }<a name="line.101"></a>
<FONT color="green">102</FONT>                }<a name="line.102"></a>
<FONT color="green">103</FONT>            }<a name="line.103"></a>
<FONT color="green">104</FONT>    <a name="line.104"></a>
<FONT color="green">105</FONT>            return result;<a name="line.105"></a>
<FONT color="green">106</FONT>        }<a name="line.106"></a>
<FONT color="green">107</FONT>    <a name="line.107"></a>
<FONT color="green">108</FONT>        /**<a name="line.108"></a>
<FONT color="green">109</FONT>         * Creates and initializes the object managed by this {@code<a name="line.109"></a>
<FONT color="green">110</FONT>         * LazyInitializer}. This method is called by {@link #get()} when the object<a name="line.110"></a>
<FONT color="green">111</FONT>         * is accessed for the first time. An implementation can focus on the<a name="line.111"></a>
<FONT color="green">112</FONT>         * creation of the object. No synchronization is needed, as this is already<a name="line.112"></a>
<FONT color="green">113</FONT>         * handled by {@code get()}.<a name="line.113"></a>
<FONT color="green">114</FONT>         *<a name="line.114"></a>
<FONT color="green">115</FONT>         * @return the managed data object<a name="line.115"></a>
<FONT color="green">116</FONT>         * @throws ConcurrentException if an error occurs during object creation<a name="line.116"></a>
<FONT color="green">117</FONT>         */<a name="line.117"></a>
<FONT color="green">118</FONT>        protected abstract T initialize() throws ConcurrentException;<a name="line.118"></a>
<FONT color="green">119</FONT>    }<a name="line.119"></a>




























































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